This invention relates to mixers in general and more particularly to a monolithic double balanced mixer having a high third order intercept point and employing an active distributed element balun.
Mixers are widely employed in the microwave art and many examples of suitable mixing circuits exist. A microwave mixer converts an RF signal to an intermediate frequency (IF) signal somewhere between the RF and a local oscillator (LO) signal. The IF signal is usually chosen to fall in the range of 20 to 100 mHz. This frequency is low enough to build high quality IF amplifiers relatively inexpensively and yet high enough to avoid the flicker noise that is inversely proportional to frequency. The IF frequency is obtained by injecting in addition to the RF signal a local oscillator (LO) or a pump frequency (.omega..sub.t).
The difference or IF frequency is a result of the mixers non-linearity. For example, certain mixers employ diodes. Hence, the diode produces the sum and difference frequencies of the two input signals when the two input signals are multiplied together. This operation of nonlinear devices in mixer circuits is well known.
As is also well known, mixer designs generally fall into three catagories: single ended, balanced, and double balanced. The double balanced mixers are capable of isolating both the signal (RF) and the local oscillator (LO) voltages from the output. Hence, the term double balanced is used. Many configurations exist in the prior art and a very popular configuration is known as the "ring circuit" which employs four diodes where the diodes are pointed in the same direction.
Another prior art configuration is called a "star circuit" which employs two diodes pointing toward the central node and two diodes pointing away from the central node. During one half of the local oscillator cycle, half the diodes are in the high resistance state and half the diodes are in the low resistance state. During the other half of the local oscillator signal, the diodes are in the opposite state. Thus, the mixer can be considered as a symmetrical switch turning on and off at the local oscillator frequency. On the average, the signal voltage at the output is zero.
For examples of prior art mixing circuits, reference is made to a text entitled Microwave Semiconductor Circuit Design by W. Alan Davis, published by Van Nostrand Rheinhold Co., 1984. Reference is made to Chapter 12 entitled "Schottky-Barrier Diode Application" pages 253-263 entitled "Mixers".
As is known in the prior art, the third order intermodulation performance of a down converter is usually limited by the performance of the mixer. Traditional diode mixers as indicated above require large LO signal levels to obtain a high IP3 which is not practical in many systems, especially in monolithic microwave integrated circuits (MMIC). It is known that a double balanced mixer will have superior IP3 performance over a signal ended or single balanced mixers. Commonly, diodes have been used as mixing elements and more recently active FETS. Neither of these will give an IP3 greater than +30 dBm even when employing a double balanced mixer configuration with low conversion loss.
The prior art has widely investigated such mixing circuits. Reference is made to an article entitled "A GaAs MESFET Balanced Mixer With Very Low Intermodulation" by Stephen A. Maas, published in the 1987 IEEE MTT-S Digest, pages 895-898. This article describes a balanced resistive mixer which was fabricated using the unbiased channel of a GaAs MESFET as the mixing element. Because the resistance is only very weakly non-linear, very low intermodulation results. The state of the art second and third order output intercept points of 34 and 21 dBm were achieved with a 7 dB conversion loss at X-band frequencies. The circuit employed an LO balun which was coupled to the gate electrodes of two FET devices where the output electrodes of the FET devices were coupled to an IF filter and transformer all of which were employed utilizing monolithic microwave integrated circuit (MMIC) techniques.
Reference is also made to an article entitled "Performance of Arrays of SIS Junctions in Hetrodyne Mixers" by Denis-Gerard Crete, et al. published in the IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-35 No. 4, April 1987, pages 435-440. This article describes milimeter wave hetrodyne mixers which employ arrays of SIS tunnel junctions. The SIS tunnel junction was demonstrated as being an extremely sensitive mixing element for milimeter wave receivers. The article thus describes such junctions as employed in mixer configurations.
For a general background in regard to the use of GaAs IC's as employed for implementing radio receivers and utilizing MMIC approaches, reference is made to an article entitled "GaAs IC Applications in Electronic Warfare, Radar and Communication Systems" by D. G. Fischer, published in the Microwave Journal, May 1988, pages 275-292. On page 280 of this article there is shown a quadrature IF down converter employing a mixer.
See also an article entitled "BroadBand MMIC's for System Applications" by Robert W. Bierig, et al. published in the Microwave Journal, May 1988, pages 261-270. This article also describes MMIC technology as well as components utilized with such technology. The article describes the implementation of mixers for microwave applications on page 268. Essentially, implementation of the broadband frequency converter function in MMIC form as indicated in the article represents a significant challenge. The article indicates that one approach that shows promise uses dual gate FETS in a distributed circuit topology. A distributed mixer employs the input capacitance of FET gates and a high impedance series transmission line section of impedance. Several dual gate FETS are cascaded to form a broad band structure for both the LO and RF mixer ports. A schematic diagram of the mixers is shown in FIG. 12A of the article. It is indicted that the circuit shown has been fabricated as an MMIC with the IF matching element consisting of a single gain stage amplifier.
Reference is also made to an article entitled "2 to 8 GHZ Double Balanced MESFET Mixer with +30 dBm Input Third Order Intercept" by S. Weiner, et al. published in the 1988 IEEE MTT-S Digest, pages 1097-1099. This article shows mixers which have a typical third order input intercept (IP3) of +30 dBm which was achieved from 2 to 8 GHZ utilizing a double balanced MESFET mixer operating in the unbiased or passive mode with +23 dBm LO input power.
A quality factor was defined in the article to show the efficiency of third order intercept to available LO power. Also a single balanced MESFET mixer from 4 to 18 GHZ with IP3 greater than +25 dBm was also described in the article.
As indicated, while diode mixers have been used as mixing elements as well as active FETS, neither of these will give an IP3 of greater than +30 dBm in a double balance mixer configuration without substantial loss.
It is, therefore, an object of the present invention to provide an improved double balanced mixer with high third order intercept point utilizing MMIC techniques.
It is a further object to provide an improved double balanced mixer employing GaAs MESFETS and using an active distributed balun.